Programmable pulse generating systems are used to treat chronic pain by providing electrical stimulation pulses from an electrode array placed in or near a patient's spine. Such Spinal Cord Stimulation (SCS) is useful for reducing pain in certain populations of patients. SCS systems typically include one or more electrodes connected to an External Pulse Generator (EPG) or an Implanted Pulse Generator (IPG) via lead wires. In the case of an EPG, the lead wires must be connected to the EPG via an exit from the body. The pulse generator, whether implanted or external, generates electrical pulses that are typically delivered to the dorsal column fibers within the spinal cord through the electrodes which are implanted along or near the epidural space of the spinal cord. In a typical situation, the attached lead wires exit the spinal cord and are tunneled within the torso of the patient to a sub-cutaneous pocket where the IPG is implanted, or the wires exit the patient for connection to the EPG.
Neural stimulators for SCS to date have been limited to waveform shapes dictated by their circuitry. Most emit relatively simple rectangular or trapezoidal stimulation phases with exponential, clamped-exponential, or rectangular charge recovery phases. Similar waveform limitations typically exist for stimulators used in other medical applications such as cardiac implants, cochlear implants, etc. Nevertheless, it is desirable that other waveform shapes be available, as such shapes may be useful in controlling which nerve fibers respond to a stimulation pulse. By selecting particular fibers for response, the therapeutic benefit of neural stimulation can be increased and side-effects decreased. Additionally, other waveform shapes may achieve effective stimulation results while requiring less energy than traditional waveforms, thereby extending the battery life of the stimulator.
Currently, the principal way to select a stimulation waveform was to design a stimulator that emitted that waveform as its only form, or as one of a handful of parameter-driven options. For example, a stimulator may be provided that adjusts the charge recovery phase to either an exponential or a rectangular shape based on the pulse rate selected by the user. The stimulation phase is typically provided with only a fixed rectangular shape. In contrast, it would be useful to allow the use of an arbitrary waveform shape for the either the stimulation phase, charge recovery phase, or both, rather than being limited to a simple waveform shape designed into the circuitry.
Desired is a capability of producing complex waveform shapes, whether or not they are inherently piecewise-linear, in particular with an ability to adjust amplitudes and pulsewidths simply and efficiently, such as, for example, by providing an ability to rescale portions of a waveform in time, thus adjusting pulsewidths on the fly without having to re-compute the waveform samples.
An additional problem with existing neural stimulators is that they include DC blocking capacitors to balance charges and prevent the flow of direct current to the body tissue. Balancing such charges and blocking dc current flow is considered desirable for safety reasons. Desired is an approach that removes or reduces the need for such blocking capacitors or their size by inherently balancing the stimulation waveform charges and preventing the flow of direct current.
Finally, it would be useful to scale an arbitrary waveform in time and amplitude in a manner that preserves the overall charge that is provided by the waveform when used in a stimulation pulse.